When light strikes a photosensitive material, light energy, or photons, are absorbed by the material, and an electrical current, often called a photocurrent because it results from the absorption of photons, is generated in the material. Photocurrent refers to the flow of electrons or holes within the material. Moreover, a peak in the photocurrent spectrum of the material which results from the absorption of photons is oftentimes referred to as an exciton.
In the prior art, it has been shown that a multiple quantum well (MQW) diode can be used as a photodetector whose voltage of maximum photocurrent, i.e., a large exciton, is dependent upon the wavelength of incident light. In this regard, see P. H. Wood et al., "Wavelength-Selective Voltage-Tunable Photodetector Made from Multiple Quantum Wells," Appl. Phys. Lett., vol. 47, no. 3, pp. 190-192, Aug. 1, 1985. An MQW material is essentially a narrow bandgap material which has a thickness substantially less than the diameter of an exciton and which is sandwiched between wider bandgap materials so as to enhance the exciton via confinement of the energy potential well. The voltage of maximum photocurrent within the MQW diode can be located and related to the wavelength of the incident light, thereby allowing for measurements of the wavelength. Identifying the wavelength of incident light is beneficial in imaging and spectroscopy applications.
However, the light incident on a photosensitive material can create multiple peaks in the photocurrent spectrum, that is, more than one exciton. The photocurrent exhibits a relatively large exciton, called a heavy hole (hh) exciton, at the particular wavelength of incident light, as well as any number of smaller excitons, called light hole (lh) excitons, at other wavelengths of the photocurrent spectrum. Needless to say, the occurrence of multiple excitons causes potential ambiguity when one attempts to determine the wavelength of incident light.
More specifically, at room temperature (i.e., approximately 300 degrees Kelvin), the selectivity of an MQW diode is step-like, producing photocurrent at all photon wavelengths at and above the exciton wavelength. If no electrical bias is applied to the MQW diode, the peak-to-valley ratio of the hh exciton to the lh exciton is typically about 1.3 to 1 and reduces considerably with applied bias. See A. M. Fox et al., "Quantum Well Carrier Sweep Out; Relation to Electroabsorption and Exciton Saturation," IEEE Journal of Quantum Electronics, vol. 27, p. 2281 (1991). As the foregoing ratio is reduced, the lh excitons become indistinguishable from the hh excitons. With a 10 V/.mu.m bias, the photocurrent peak at the lh exciton is actually larger than the peak at the hh exciton and occurs at the same wavelength as the hh exciton at 0 V/.mu.m bias.
Thus, a heretofore unaddressed need exists in the industry for a tunable monolithic photodetector for detecting light at a selected wavelength within a wavelength range and which does not suffer from the problems and deficiencies of the prior art, as noted in the foregoing.